Aging alkali cellulose



June 29, 1954 PPM COBALT R- L MITCHELL AGING ALKALI CELLULOSE Filed May 22, 1952 INEFFECTIVE REI JU ON IN AGING REQUIREMENT ZALPHA CELLULOSE CONTENT OF PULP INVENTOR Ferd layer; J/Ifcfid/ ATTORN Patented June 29, 1954 AGING ALKALI CELLULOSE Reid. Logan MitchelL-Shelton, Wash., assig nor to Rayonier Incorporated, Sheltn,.Wash., a 01!- poration of Delaware Application-May 22, 1952, Serial No. 289,259

This invention relates to the viscose process and has for its object the provision of certain improvements in the aging of .the alkli cellulose for use in the manufacture of viscose. More particularly, the invention is concerned with the use of cobalt as a catalyst for the depolymerization of the cellulose to decrease its viscosity (commonly called aging In accordance with the invention, a critically small amount of cobalt, preferably correlated to the alpha-cellulose content of the pulp, is incorporated in the pulp to control aging.

Various suggestions have been made heretofore to add cobalt to wood pulp and also to alkali cellulose to diminish the aging time. the use of cobalt for this purpose has not been satisfactory due to the fickleand unpredictable results. In fact, cobalt has been regarded as objectionable because it was not understood that an amount of cobalt that would give a desired aging effect in one pulp would give an entirely diiferent aging efiect in a pulp having a different alpha-cellulose content. It was further not foreseeable, from the experience in using other metals such as iron or manganese, that such a small quantity of cobalt would have such a pronounced effect on aging.

The invention is based on my discovery of a critical relationship between the alpha-cellulose content of the pulp, the per-cent reduction of aging, and the amount of cobalt incorporated in the pulp. One of the surprising aspects of the disco-very is that such extremely small amounts of cobalt exercise such pronounced, and different, effects on the aging of the alkali cellulose. In accordance with the invention, such an amount of cobalt, suitably correlated to the alpha-cellulose content of the pulp, is incorporated in the pulp to efiect a predetermined and desired reduction in the aging period of the alkali cellulose. I have found that less than p. p. m. (parts per million) of cobalt based on the bone dry weight of the pulp is an amount sufficient to give practically any desired control over aging. than this small maximum amount, care must be taken not to use such an excessive amount as to cause undue degradation of the cellulose. For most aging requirements, satisfactory results are attained with less than 5 p. p. m. of cobalt, suitably correlated to the alpha-cellulose content of the pulp and the aging effect sought. With some pulps, very effective reduction. in theaging time can be achieved with less than 1 p. p. m. of cobalt, but it is generally-advantageous However,

However, even with amounts less 1 Claim. (Cl. 260233) 2 in the interest of uniformity, due to the diificulty in dispensing such small amounts, to incorporate at least 1 p. p. m. of cobalt in the alkali cellulose pulp. The cobalt may be added to the pulp or to any material used in the preparation of the alkali cellulose, as for example, to. the Wood pulp, to the caustic soda or to a mixture of these when in a slurry. The cobalt is preferably added in the form of a water soluble compound such as cobaltous sulfate, nitrate, or chloride, etc.

The references herein to the amount of cobalt in p. p. m. refer to the contained elemental cobalt and not to the compound, and the amounts of cellulose are on the basis of bone dry cellulose.

The accompanying coordinate diagram illustrates the critical effect of cobalt on the aging of alkali cellulose formed of pulps containing different percentages of alpha-cellulose such as are used for conversion to alkali cellulose in the viscose process.

Alpha cellulose content is one of the most commonly used measures of pulp purity; among other things it (1) reflects, particularly for wood pulps, the amount of alkali used in refining which in turn is related to degree of mercerization and potential ease of depolymerization of the finished pulp, and (2) parallels the absorptive affinity and cleavage sensitivity of the pulp to cobalt.

The term alkali cellulose aging or aging requiremen as used herein. refers to the period of storage subsequent to shredding and prior to xanthation in which sufficient depolymerization is accomplished so that the resultant viscose will have normal spinning viscosity.

It will be noted by reference to the chart that, say, 0.25 p. p. m. of cobalt will effect a 75% reduction in the aging of 98% alpha-cellulose, but only a reduction in aging of 95% alphacellulose. It will be noted also by reference to the reduction curve that a 92% alpha-cellulose requires 2 p. p. m., while a 97% alpha-cellulose requires but 0.3 p. p. m. to reduce the aging period to the same amount. Similar other Wholly unpredictable comparative effects are clearly evident. In accordance with the preferred and most advantageous embodiment of the invention, I incorporate in the alkali cellulose the amount of cobalt Within the shaded area of the chart, preferably correlated with the alpha-cellulose content of the pulp and the per-cent of aging desired.

The cobalt may be incorporated in the reacting materials in any suitable wayprovided it is present in the alkali cellulose, and especially Where a shredding operation is used because additional beneficial results will occur in shredding. However, there is an important advantage to be gained in the way of the uniformity of results by introducing the cobalt in the pulp since cobalt is tenaciously adsorbed and stripped from caustic soda steeping liquor by cellulose, and it is diificult in commercial operation to control satisfactorily the amount of cobalt in the steeped alkali cellulose by dissolving cobalt salts in the steeping liquor. While the cobalt thus added may be effective in lowering viscosity, the important thing is not merely to lower viscosity but to lower it in a continuously uniform manner.

The cobalt additions may be applied to the cellulose by any suitable method ensuring uniformity in the level of added cobalt. In one method of application I prefer to spray a dilute solution of cobalt sulfate onto the formed pulp sheet as it it passes into the dryer on the pulp machine. In another method I prefer to apply a solution of cobalt sulfate at a uniform level as A. inch strips along the machine direction of the formed pulp sheet, either while in the pulp dryer or later in the cutter used to cut rolls into sheets.

Since added cobalt in the extremely small amounts of 1-10 p. p. m. is not visible on the pulp sheet it is sometimes desirable to add a suitable dye to the cobalt treating solution in such amount (1 part dye to 1000 parts cobalt is sufficient for Victoria Green) as to label and identify the treated pulp.

The following examples illustrate methods carried out in accordance with the invention in the preparation of viscose from refined wood pulp in which DP refers to degree of polymerization:

Example I A web of highly refined wood pulp (780 DP, A

pressed to a press ratio of 2.70 and shredded for 1 hour at 30 C. The alkali cellulose crumb was aged for hours at 30 C. It was then xanthated for 1 hours using 31% CS2 based on cellulose in alkali cellulose. The xanthate was mixed in a dissolver for 2 hours at 15 C. to give a viscose having the composition of 8.5% cellulose and 5.25% sodium hydroxide. After ripening for 24 hours at C. the resulting viscose had a viscosity of 35 sec. falling-ball (5,300 centipoises) (300 DP) a salt index of 7.5 and gave a filtration plugging value of 1500 g./sq. cm. A sample of identical pulp but without the cobalt addition, processed in analogous manner, required hours aging at 30 C. to give the same: viscose viscosity. Viscose filtration for this latter pulp was 1400 g./'sq. cm. Tensile and durability properties or" films cast from the two viscoses were equivalent.

Example II Sheets of cellulose pulp (780 DP, 88% alphacellulose) stripped With a solution of oobaltous sulfate in such manner as to contain 5 p. p. m. cobalt based on the bone dry weight of the pulp were steeped for 30 minutes at 30 C. in an equilibrium caustic soda liquor containing 1.5% hemicellulose and 18.5% sodium hydroxide, pressed to 2.60 press ratio, shredded for 60 minutes at 30 C. aged for 24 hours at 30 C., Xanthated for 1 hours at 30 C. using 30% CS2, mixed for 2 hours at 15 C. to give a viscose containing 10% cellulose and 5% sodium hydroxide, then allowed to ripen for 24 hours at 20 C. The viscose had a viscosity of 35 sec. falling-ball (5,300 centipoises) (185 DP) a salt index of 6.5 and gave a filtration value of 1000. In the preparation of a similar viscose from pulp containing no added cobalt an aging time of 72 hours at 30 C. was required. Both viscoses had similar filterability and gave film of equivalent quality.

Example III Sheets of a cellulose pulp (800 DP, 89% alphacellulose) containing 2 p. p. m. cobalt were steeped in a caustic soda steeping liquor containing 1.5% hemicellulose and 18.0% sodium hydroxide, pressed to 2.60 press ratio, shredded for 1 hour at 30 C., aged 18 hours at 30 C, xanthated for 1 hours at 30 C. using 28% CS2 based on the cellulose in alkali cellulose, mixed to give a viscose containing 8.5% cellulose and 4.0% sodium hydroxide, then ripened for 24 hours at 20 C. The viscose had a viscosity of 40 seconds falling-ball (6,000 centipoises) (285 DP) a salt index of 5.5 and a filtration value of 750. In the preparation of a similar viscose from identical pulp containing no added cobalt, the alkali cellulose aging requirement was 36 hours at 30 C.

Example IV Sheets of a pulp (1150 DP, alpha-cellulose) containing 0.5 p. p. m. added cobalt were steeped in a caustic soda steeping liquor containing 1.0% hemicellulose and 19.0% sodium hydroxide, pressed to a 2.50 press ratio, shredded for 1 hour at 30 C., aged for 10 hours at 30 0., xanthated for 2 hours at 30 C. using 34% C82, mixed to give a viscose containing 7.5% cellulose and 6.5% sodium hydroxide, then ripened for 24. hours at 20 C. The viscose had a viscosity of 45 sec. falling-ball (6.700 centipoises) (400 DP) a salt index of 9.5 and a filtration value of 3500. In the preparation of a similar viscose from identical pulp containing no added cobalt, an aging time of 30 hours at 30 C. was required. The filtration value of the latter viscose was 3200. Tire cords prepared from the two viscoses had equivalent quality factor and fatigue life.

Example V Sheets of pulp (1050 DP, 93% alpha-cellulose) were treated to contain 0.5 p. p. In. of added cobalt. The aging requirement was then equivalent to that for a 600 DP pulp although the actual DP was unchanged by the treatment.

Example VI Sheets of pulp (2000 DP, 96.5% alpha-cellulose) were treated to contain 0.5 p. p. m. of added cobalt. It aged like a pulp of 1150 DP.

With 1 p. p. m. of added cobalt it aged like a pulp of 780 DP.

With 10 p. p. m. of added cobalt it aged like a pulp of 600 DP.

Various compounds of cobalt may be used. For reasons of economy and convenience in application, the water soluble inorganic cobalt compounds, such as cobaltous sulfate or nitrate, are very desirable.

Emample VII Pulp of 96% alpha, 1650 DP, treated to contain l p. m. cobalt added as a narrow strip (dyed green with Victoria Green, 0.001 p. p. m. on cellulose) was steeped in equilibrium steeping liquor containing 1.0% hemicellulose and 18.5% sodium hydroxide for 1 hour at 25 C., pressed to 2.60 ratio, shredded 1 hour at 30 C., aged for 7 hours at 30 C., xanthated for 2 hours at 30 C. using 341% CS2, mixed to give a viscose containing 7.5% cellulose and 6.5% sodium hydroxide,

then ripened for 24 hours at 20 C. The viscose had a viscosity of 35 sec. fall ng-ball (5300 centipoises) (360 D?) a salt index of 9.0 and a filtration value of 2800. In the preparation of a similar viscose from identical pulp containing no added cobalt, an aging time of 46 hours at 30 C. was required.

In the preparation of similar viscose from identical pulp containing 0.2 p. p. in. added cobalt the aging requirement was 22 hours at 30 C.

In the preparation of similar viscose from identical pulp containing 1 p. p. m. added cobalt by a similar process except that shredding was for 2 hours at 50 no aging time was required, suflicient depolyinerization having been accom plished during shredding. Under these same conditions, pulp without added cobalt required an aging time of 2 hours at 30 C.

Example VIII Pulp of 97% alpha, 1550 DP was treated to contain 1 p. p. m. of added cobalt giving it an aging requirement of 5 hours for conversion into rayon type 7.5-6.5 viscose. Identical pulp without the added cobalt had an aging requirement of 42 hours when processed into similar viscose.

Example IX Pulp of 94.9% alpha and 1150 DP was treated to contain added cobalt in amounts of 0, 0.1, 0.5, l, 2, 5 and 10 p. p. in. When converted to rayon type viscose the aging requirements for these pulps were found to be respectively 30, 1'7, 10, 6, 3.5, 2.5 and 2 hours at 30 C.

Example X A cold caustic refined pulp of 99.3 alpha and 1100 DP was treated to contain 0.2 p. p. m. of added cobalt. When processed into tire cord type viscose the aging requirement was found to be 8 hours at 36 C. With no added cobalt the aging requirement was 24 hours at 30 C.

I claim:

The improvement in the viscose process which comprises aging alkali cellulose prepared from cellulose pulp containing from 83% to 99% of alpha cellulose with the amount of cobalt, based on the bone dry weight of the cellulose pulp, falling within the shaded area and substantially along the ordinate of the alpha cellulose content of the cellulose pulp of the graph of the accompanying drawing.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,542,285 Mitchell Feb. 20, 1951 2,542,492 Entwistle et al. Feb. 20, 1951 OTHER REFERENCES Cellulose and Cellulose Derivatives (O'tt) Published by Interscience Publishers, Inc., N. Y. C., 1943. (Page 744 relied on.) 

